1. Field of the Invention
The present invention relates to a backlight for a color liquid crystal display device, a color liquid crystal display device, and an EL element for a backlight of a color liquid crystal display device.
2. Description of the Related Art
Color liquid crystal display devices are used as display devices for computers, game machines and various kinds of home appliances. With the recent increase of various kinds of portable information terminals, a large number of liquid crystal display devices are used as display devices for the information terminals. In particular, also in PDA""s (personal digital assistants) and portable telephones, color display has been forwarded. Accordingly, demands are strong for color liquid crystal display devices smaller and thinner in size, and lower in power consumption.
The liquid crystal display device, being a display device that does not emit light, is ordinarily provided with a backlight as a light-emitting source. For the backlight of the color liquid crystal display, cold-cathode tube is generally used. However, the cold-cathode tube, in addition to being large in power consumption, requires a large space for installation. As a result, it cannot be employed as the display portion of the portable information terminals such as PDA""s and portable telephones. That is, for the display portion that requires smaller and thinner size and lower power consumption, a color liquid crystal display device having a backlight consisting of a cold-cathode tube cannot be applied.
From these reasons, in the portable information terminals, liquid crystal display devices having light-emitting diodes (LED""s) or electroluminescent (EL) light-emitting elements are used. However, the LED, being a point light source, has disadvantages such as the tendency to cause unevenness or light leakage when displaying in a larger screen, or breakage under high load operation. Furthermore, when the LED is used as the backlight, a plurality of LED""s are necessary according to a dimension of a display screen, a rise of the device cost being inevitably caused.
On the other hand, the EL element, being a surface light source, does not cause unevenness even when displaying in a larger screen, and furthermore, being light and thin to be high in freedom of shape, is excellent in space saving and lower in power consumption. Thus, since the EL element is superior as the backlight for the portable information terminal, a liquid crystal display device using an EL element as the backlight has been studied to apply in a portable information terminal (cf. Japanese Patent Laid-Open Application Nos. JP-A7-43712 and JP-A 11-211864, for instance).
In an ordinary EL element, a copper activated zinc sulfide phosphor is used as an EL phosphor. However, the copper activated zinc sulfide phosphor can emit only in from blue green to green color, resulting in inferior color reproducibility. Furthermore, the EL element is insufficient in brightness in comparison with the LED. In particular, white light of high brightness is necessary for the backlight of the color liquid crystal display device. However, the existing EL element is insufficient in white light reproducibility and brightness, as a result the EL element has never been put into practical use as the backlight of the color liquid display device.
For instance, in the above Japanese Patent Laid-Open Application No. JP-A 7-43712, as a backlight for a color liquid crystal display device, the use of a white-emitting EL lamp is disclosed. However, an EL light-emitting layer used here is formed by means of electron beam evaporation method, sputtering method, or CVD method, being different in its configuration from an EL element of organic dispersion type. Furthermore, on the basis of the difference of the configuration of the EL light-emitting layer or the like, a backlight disclosed in the above gazette cannot give white light of sufficient brightness.
On the other hand, in Japanese Patent Laid-Open Application No. JP-A 11-211864, it is disclosed that a color EL lamp in which a layer of a fluorescent pigment is formed as a coloring layer on a surface of the EL lamp is used as a backlight for a liquid crystal display device. The color EL lamp here is one to obtain a single color emission and is different in configuration from a white-emitting EL element demanded as a backlight for a color liquid crystal display device. Accordingly, with the color EL lamp disclosed in the above gazette, full-color display due to a liquid crystal display device cannot be realized.
Accordingly, an object of the present invention is to provide a backlight for color liquid crystal that, in addition to enabling to realize a high performance full color display due to a liquid crystal display device, is smaller and thinner to be excellent in space saving, lower in consumption power, and free from display unevenness and light leakage, and an EL element for a backlight of color liquid crystal. Furthermore, the other object of the present invention is, by the use of such backlight, to provide a color liquid crystal display device in which display unevenness and light leakage are prevented from occurring and of which display characteristics such as color reproducibility and brightness are improved.
A backlight for color liquid crystal of the present invention comprises a light-emitting layer, a transparent electrode layer and an EL element. In the above, the light-emitting layer comprises EL phosphor particles dispersed in a matrix of high dielectric constant. The transparent electrode is disposed along a main surface on a light-emitting side of the light-emitting layer. The EL element comprises a reflective insulating layer and a rear surface electrode layer stacked in turn along a main surface on a non-emitting side of the light-emitting layer. Here, the EL element, under operating conditions of a voltage of 100 Vrms and a frequency of 400 Hz, emits white light of brightness of 80 cd/m2 or more and has a luminous efficiency of 30 W/m2 or less in consumption power.
An EL element for a backlight of color liquid crystal of the present invention comprises a light-emitting layer, a transparent electrode layer, and a reflective insulating layer and a rear electrode layer. In the above, the light-emitting layer comprises EL phosphor particles dispersed in a matrix of high dielectric constant. The transparent electrode is disposed along a main surface on a light-emitting side of the light-emitting layer. The reflective insulating layer and rear electrode layer are stacked in turn along a main surface on a non-emitting side of the light-emitting layer. Here, the EL element, under operating conditions of a voltage of 100 Vrms and a frequency of 400 Hz, emits white light of brightness of 80 cd/m2 or more and has a luminous efficiency of 30 W/m2 or less in consumption power.
The EL element having the white light emission as mentioned above satisfies characteristics (reproducibility of white light and brightness) necessary for the backlight for liquid crystal. Accordingly, by the use of such EL element as the backlight, excellent color display due to a liquid crystal display device can be realized. That is, the color liquid crystal display device in which the EL element is used as the backlight can be put in practical use.
In addition to the above, by making the best use of the characteristics of the EL element of being small and thin, that is, excellent in space saving, and low in power consumption, the color liquid crystal display device can be made smaller and thinner in size, and lower in power consumption. Furthermore, the EL element, being a surface light source, does not cause display unevenness and light leakage. Accordingly, the color liquid crystal display device can be furthermore improved in its display characteristics.
In the present invention, for the EL phosphor, a copper activated zinc sulfide phosphor that emits mainly in blue green color (or blue color) is applied. In order to make the brightness of white emission due to the EL element 80 cd/m2 or more, a copper activated zinc sulfide phosphor that has brightness of 100 cd/m2 or more under the operating conditions of for instance a voltage of 100 Vrms and a frequency of 400 Hz is employed. In addition to the use of such high brightness copper activated zinc sulfide phosphor, the following configuration of the EL element is adopted to obtain white emission. Thereby, the white emission excellent in color reproducibility and high in brightness can be realized.
A specific configuration of an EL element for obtaining white emission is as follows. That is, a light-emitting layer of the EL element is preferable to comprise copper activated zinc sulfide phosphor particles and a fluorescent pigment. Here, the copper activated zinc sulfide phosphor particles emit in blue green or blue color. The fluorescent pigment absorbs emission from the copper activated zinc sulfide phosphor to emit in red color and is contained by 3 mass percent or more with respect to the copper activated zinc sulfide phosphor. In such light-emitting layer, the red fluorescent pigment is more preferable to be contained in the light-emitting layer in the range from 4 mass percent or more to 7 mass percent or less with respect to the copper activated zinc sulfide phosphor.
In the present invention, as another configuration of the EL element having white emission, a light-emitting layer of an EL element comprises copper activated zinc sulfide phosphor particles and a fluorescent pigment. Here, the copper activated zinc sulfide phosphor particles emit in blue green color or blue color. The fluorescent pigment absorbs emission from the copper activated zinc sulfide phosphor to emit in red color and is contained less than 2 mass percent with respect to the copper activated zinc sulfide phosphor. The EL element further comprises a red pigment layer. The red pigment layer is stacked on a transparent electrode film thereon the transparent electrode is formed and contains the fluorescent pigment of from 2 mass percent or more to 5 mass percent or less with respect to the copper activated zinc sulfide phosphor in the light-emitting layer.
A color liquid crystal display device of the present invention employs the EL element as mentioned above as a backlight. That is, the color liquid crystal display device of the present invention comprises the aforementioned backlight for the color liquid crystal of the present invention and a transmissive or reflective liquid crystal display element. The transmission or reflection type color liquid crystal display element is disposed on a light-emitting surface side of the aforementioned backlight.
In the color liquid crystal display device of the present invention, in order to increase an amount of light supplied to the color liquid crystal display element, it is effective to dispose, between the backlight and the color liquid crystal display element, a reflective optical retardation film to improve utility efficiency of white light emitted from the EL element. Furthermore, as a drive power source of the EL element, an inverter having an output of 150 Vpp or more and 300 Hz or more is used to enhance furthermore brightness of the white emission.